The invention relates to an air-to-air heat exchanger for ventilation systems.
The insulation values required of building envelopes under current building regulations relative to heated buildings are such that controlled ventilation is a necessity. Such ventilation serves to transport stale air, i.e. air carrying pollutants and moisture, to the outside, replacing it with fresh, oxygen-filled exterior air. This used to be accomplished by means of regular ventilation via windows and doors, although this often cannot be done to a sufficient degree, for example when residents are absent for any length of time. Furthermore, such ventilation replaces the heated air on the inside with cold air from the outside, which in turn requires energy to heat the fresh air. For reasons of cost and ecology, this is not desirable.
Various types of ventilation installations based on heat recovery have long been in use. Part of the energy content of the heated waste air is transferred to the incoming fresh air. This is generally achieved using cross-flow heat exchangers or rotary heat exchangers, both of which are very complex in their construction, and hence relatively expensive. Reverse air ventilators are another variant, where a fan changes its direction of rotation at regular intervals, and therefore the direction in which air is conveyed, ensuring that a proportion of stale indoor air is first blown outside and then replaced with fresh air drawn inwards.
Counterflow heat exchangers are disclosed in, for example, DE 10 2006 051 903 A1, DE 10 2006 035 531 A1, DE 10 2005 045 734 A1, DE 10 2005 035 712 A1 and DE 10 2004 046 587 and EP 2 077 428 A2. German patent application DE 10 2008 058 817 A1, which represents the closest prior art, discloses an air-to-air heat exchanger operating according to the countercurrent principle where a first air flow is guided inside closed pipes whilst a second air flow, which flows counter to the first air flow, is located in an intermediate space between the pipes and the cylindrical exterior housing. For the purpose of moving the countercurrent air flows there is a fan disposed on one end of the cylindrical housing, comprising an inner ring and an outer ring disposed concentrically around the inner ring to transport air in the opposite direction. The spaces occupied by the outer ring and the inner ring are separated from each other by a cylindrical wall. In one embodiment, the pipes leaving the fan initially diverge conically, then run parallel and finally converge conically. In this way the second air flow circulates around the pipes in the intermediate space between the pipes, thereby allowing an efficient exchange of heat.
The design of this heat exchanger is relatively complex, however, and correspondingly costly to produce. In addition, further ways of improving the efficiency of this heat exchanger are still being sought.